There is a fixing device that fixes a toner image on a sheet of paper by heating and pressing as a fixing device incorporated into an image forming apparatus using an electro-photographic process, such as a copying machine, a facsimile machine, and a printer. Such a fixing device fixes a toner image by heating and pressing by inserting a sheet of paper through a nip formed between a pair of rollers comprising a heat roller and a press roller or between belts of the same kinds. Meanwhile, a full color toner image formed by superimposing toner images of plural colors has a toner layer several times thicker than that of a monochromatic toner image. With this being the case, generally, when a monochromatic toner image is fixed, only the heat roller that comes into contact with the toner image is heated without heating the press roller. On the other hand, when a full color toner image is fixed, both the press roller that supports the back surface of a sheet of paper and the heat roller that comes into contact with the toner image are heated.
In other words, when a color toner image is fixed, power is supplied to both the heat roller and the press roller whereas power is supplied to the heat roller alone when a monochromatic toner image is fixed. With this being the situation, there has been a device that supplies, when a monochromatic toner image fixed, the heat roller with power increased by an amount comparable to the power that is supplied to the press roller when a color toner image is fixed. This device in the related art shortens a warm-up time by increasing power to be supplied to the heat roller when a monochromatic toner image is fixed, and it further increases a speed when a monochromatic toner image is fixed by keeping a desired fixing temperature even when fixing is performed successively. In addition, when a full color toner image is fixed, it heats the bottom layer of a toner image by supplying power to the press roller and thereby achieves a satisfactory fixing performance.
The fixing device in the related art, however, uses a heater lamp as a heat source of the heat roller and the press roller. For such a heater lamp, power consumption is normally determined by the heater lamp. Hence, in a case where an amount of power used for the heater of the press roller is added to the power on the heat roller side, a warm-up heater lamp is actually added on the heat roller side. Moreover, in order to keep a constant temperature on the surface of the heat roller, plural heater lamps need to be disposed at regular intervals. This makes it difficult to achieve a size reduction of the heat roller.
Meanwhile, a total amount of power that can be used for the entire color image forming apparatus is fixed to 1500 W. The amount of power actually available for a heat source of the fixing device is therefore found to be a remaining amount of power, which is a remainder when an amount of power used for the driving source, such as motors, and an amount of power used for optional functions, such as finisher, are subtracted from 1500 W. Moreover, power consumption of the heat source of the fixing device is set so that the fixing device can be incorporated into an apparatus in which the power of the driving source and the power consumption of the optional function are set at a maximum.
In a case where the fixing device is incorporated into a color image forming apparatus that is not furnished with optional functions or has a small driving source, power consumption available for the heat source can be increased further. In such a case, however, power consumption of the heater lamp cannot be adjusted freely.
Under these circumstances, there has been a need for a fixing device for a color image forming apparatus not only capable of enhancing the fixing performance when a color toner image is fixed, but also capable of enhancing the fixing productivity when a monochromatic toner image is fixed by controlling the heat source of the fixing device at a higher degree of flexibility to match with the structure of the color image forming apparatus or image forming conditions, and a control method of the fixing device for a color image forming apparatus.